Most axial fans exhaust air in the axial direction. When such a fan is mounted in the rear panel of a device being cooled, care must be taken not to position that panel against a wall. Even when there is space between the panel and wall, back pressure from air striking the wall can both reduce the efficiency of the fan and also create an undesirable level of noise. Axial fans are discussed in R. J. Kenny: "Fans and Motors", Machine Design, Mar. 14, 1968, pages 158-159.
Some low-cost axial fans are equipped with an impeller made by cutting a flat metal blank into a daisy-like shape, the petals of the daisy being formed into air-driving blades. The blades of more efficient impellers, when formed from metal, may be individually welded to a cylindrical metal hub, so that each blade is supported along its full length. Such welded impellers are considerably more expensive than are those that are formed from a blank. Molded plastic impellers similar to the welded impellers can be equally efficient.
Of axial fan impellers which are known to the inventor, those that have been formed from blanks have a ratio of blade depth to radius between 0.3 and 0.4, whereas that ratio usually exceeds 0.5 in welded and comparable molded plastic impellers. Testing by the inventor indicates that ratios above 0.5 provide greater efficiency.